1. Field of the Invention
This invention relates to a method for providing stable rotation rates for a hydrostatic transmission when the transmission operates at very low speeds on the order of a few revolutions per minute or less.
2. Discussion of Related Art
In the art of measuring certain physical characteristics of the earth formations penetrated by a borehole such as an oil well, a logging sonde is lowered into the borehole from a cable to depths of many thousands or perhaps even two or three tens of thousands of feet. Many of the physical measurements are made continuously as the tool is drawn slowly past the formations of interest. The time base of the measurements is a function of the speed of tool displacement and depends critically on the uniformity of tool movement.
Many measurements depend on the flight time of an interrogation signal that is launched into the formation adjacent to the tool. The interrogation signal solicits a response signal from the formation so that the tool must, in effect, dwell in situ awaiting the response. In actual practice, the tool is configured such that the interrogation device, that is, the transmitter, is spaced apart from and above a sensor device by some appropriate finite distance. The tool is moved upwards, for example, at a desired speed the so that sensor will move into a position opposite the original interrogation location by the time that the response signal actually emerges from the formation. That desideratum demands that the tool be moved at an accurately-selected speed that is appropriate to the type and timing of the measurements being made. The logging-tool sonde speed ranges may typically vary from one or two feet per minute to several hundred feet per minute.
The logging tool is suspended in the borehole from a logging cable that serves both as a support means for the tool and as a means for interchanging information, power and data between the surface processing equipment and the sensors in the tool. The cable is payed out and retrieved using a draw works or hoisting drum having a capacity of many thousands of feet of cable. Usually the hoisting drum is powered by a conventional hydrostatic drive. Hydraulic power may be provided by a desired type of prime mover such as a diesel engine. The hydrostatic drive may have a power rating on the order of 150 to 200 hp.
As explained earlier, tool deployment requires a stable speed control having a wide dynamic range. Conventional hydraulic drives, however, become unstable when the variable-displacement pump flow-rate and the rotational speed of the motor must be reduced to accommodate very low cable-drum speeds. One manufacturer (Mannesmann Rexroth), with respect to their axial piston motors, warns that " . . . if uniformity of rotation is required, however, the speed should not be allowed to fall below 50 rpm . . . " yet rotation rates of but a single rpm are often required. Part of the problem stems from the inability of the variable displacement hydrostatic pump itself to precisely control the flow at very low flow rates. Hose length, hose diameter, erratic hose expansion and fluid compression under extreme pressures all tend to worsen the problem. Rigid plumbing helps a little bit but is undesirable because it is inflexible and awkward to implement in the field.
In the well-logging industry, logging cable drums are hydrostatically powered as described. The required speed range is achieved by providing a mechanical two- or three-speed gear transmission. The obvious disadvantage to such transmissions is that the speed-change curve is not smooth; it necessarily has two or three discontinuities. That is, the logging run must be interrupted while the operator changes gears.
The assignee of this invention provides a set of change gears that involve disk-type clutches to provide continuous mechanical gear-ratio changes. That transmission has the disadvantage of being very expensive and somewhat demanding maintenance-wise.
There is a need for a pure hydrostatic transmission that is characterized by a wide dynamic range in speed control and output power and a mechanism that will be economical, easily maintained and highly reliable.